1. Field of the Invention
The invention relates to an arrangement of surfaces with an optical diffraction effect, as set forth in the classifying portion of claim 1.
Surface patterns of that kind are additionally used to enhance the level of safeguard against forgery in relation to optical-diffraction security elements which are made up of holograms and/or diffraction gratings of a mosaic-like composition.
2. Description of the Prior Art
An arrangement of the kind set forth in the opening part of this specification, in relation to optical-diffraction security elements, is known from EP-A 105 099. It describes the production of variable patterns of surface portions which have an optical diffraction effect and which are glued for example in the form of a stamp on a document and authenticate that it is genuine. When the security feature is illuminated, those surface portions are successively lit up upon rotation about an axis perpendicularly to the plane of the security feature, along a path.
PCT application WO 97/27504 discloses a surface pattern having at least two surface portions of which at least one surface portion includes a diffraction grating which is superimposed of two relief structures and which in a marked direction has the same diffraction property as a different second surface having a diffraction effect. In other directions the two surface portions exhibit a different diffraction behaviour. The two relief structures of the superimposed diffraction grating differ at least in terms of one of the parameters consisting of spatial frequency, relief shape and azimuth.
It is also known from EP-A 0 360 969 for at least one optical diffraction element of the optical-diffraction authenticity feature to be subdivided into two surface portions whose microscopically fine asymmetrical diffraction gratings only differ in terms of azimuth through 180.degree., with the other grating parameters otherwise being the same. With those subdivided diffraction elements, machine-readable information can be inconspicuously disposed in a visually perceptible pattern.
The above-listed documents describe security elements without effectively utilising brightness modulation within relatively large areas which can be well perceived by the naked human eye.
In addition EP-A 0 401 466 describes a plastic laminate with embedded, microscopically fine, optically effective relief structures and the use thereof as a security element. The materials which can be used are known for example from EP 0 201 323 B1.